Conventionally, patent literature 1 proposes an SiC semiconductor device including vertical metal-oxide-semiconductor field-effect transistor (MOSFET) having inverted type trench gate structure, as a vertical switching element having a trench gate structure. In the vertical MOSFET having the inverted type trench gate structure, a gate voltage is applied to a gate electrode in a gate trench, a channel is formed in a p-type base region located on a side surface of the gate trench, and a current is allowed to flow between a drain and a source through the channel. In such a vertical switching element having the trench gate structure, when a high electrical field is applied to a gate insulation film included in the trench gate structure, insulation breakdown is caused and element breakdown voltage is decreased.
In the SiC semiconductor device of the patent literature 1, a deep trench is formed at both sides across the trench gate structure. The deep trench penetrates an n+-type source region and a p-type base region to reach an n−-type drift layer and has a p-type deep layer in the deep trench. The p-type deep layer restricts the electrical field from breaking into the gate insulation film and protects the gate insulation film from the high electrical field. Accordingly, the insulation breakdown is less likely to be caused and the element breakdown voltage is improved.